Geographically localised shelf waves on curved coasts     

G. Kaoullas  E.R. Johnson 《Continental Shelf Research》2010

This paper presents a combined analytical and numerical extension to recent theoretical work on trapped continental shelf waves over smoothly curved coastlines. The geometry considered consists of an annular region joining two semi-infinite straight sections and, even though the isobaths remain evenly spaced, trapped modes appear in the annular region due entirely to the longitudinal curvature of the shelf there. Expanding in alongshore modes and requiring that the error is orthogonal to the transverse modes gives a linear algebraic system of equations for the frequency of trapping. Solutions for barotropic, non-divergent shelf waves show that trapped modes always exist if the coastline curves to form a headland.  相似文献   

Global Earth structure: inference and assessment   总被引：3，自引：0，他引：3  

D. W. Vasco  Lane R. Johnson  & Osni Marques 《Geophysical Journal International》1999,137(2):381-407

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Application of carbon isotope analysis to ancient maize agriculture in the Petexbatún region of Guatemala     

Kristofer D. Johnson  David R. Wright  Richard E. Terry 《Geoarchaeology》2007,22(3):313-336

The ancient Maya subsisted in an environment limited by shallow soils and unpredictable weather patterns until their collapse ∼A.D. 800–900. Ancient subsistence can be a difficult subject, with little physical evidence of agricultural artifacts and structures. This study characterized soil profiles and utilized changes in stable carbon isotope ratios of soil organic matter (SOM) to locate and interpret areas of ancient C₄ plant growth and maize (Zea mays) cultivation among the Maya. The investigation indicated some of the challenges the Maya faced, including shallow and sloped soils in some areas. The C₄ plant signature was found in seasonal wetland soils on the opposite side of the Laguneta Aguateca from the ruins of Aguateca, but not in the perennial wetlands on the immediate side. No C₄ plant signature was detected in the shoulder and backslope soils. Based on these findings, the ancient Maya of Aguateca probably adapted to their environment by farming rich toeslope soils. It is possible that maize was also grown in the seasonal wetlands adjacent to the site. If the steep backslope soils around Aguateca were used in ancient agriculture, the evidence has probably eroded away. © 2007 Wiley Periodicals, Inc.  相似文献   

中国黄土高原西缘甘肃万象洞MIS 5石笋δ18O记录与南方地区石笋记录的对比研究   总被引：1，自引：0，他引：1  

安春雷  张平中  代志波  张德忠  杨勋林  K.R.Johnson 《第四纪研究》2006,26(6)

位于青藏高原与黄土高原过渡带的甘肃武都万象洞石笋WXSM51和WXSM52提供了MIS 5(118～79 kaB.P.)高分辨率的δ18O记录.研究表明,万象洞石笋δ18O值与夏季风强度呈负相关关系,与我国西南部的贵州董歌洞石笋δ18O记录有良好的对应关系,并与高纬度的格陵兰NGRIP冰芯δ18O记录和65°N太阳辐射强度有很好的一致性,说明万象洞石笋δ18O记录了118～79 kaB.P.期间亚洲季风强度的变化,同时也说明东亚季风强度的变化和全球气候变化同步,而且主要受控于北半球太阳辐射强度的变化.同时它与地中海碳酸盐记录有很好的相似性,和巴西石笋δ18O记录在千年尺度上表现出相反的变化趋势,说明东亚季风区、地中海地区以及巴西季风区之间存在密切的联系,指示了南北半球气候在千年尺度上存在"跷跷板"(seesaw)现象.万象洞石笋δ18O记录的MIS 5b与MIS 5a突发性转换,与NGRIP冰芯δ18O记录相似,而与神农架记录存在差异,说明万象洞地区对亚洲季风强度的响应更为敏感.  相似文献   

Quaternary ecology: A paleoecological perspective. Hazel R. Delcourt and Paul A. Delcourt, 1991, Chapman & Hall,x + 242 pp., $93.00 (clothbound), $43.95 (paperbound)     

William C. Johnson 《Geoarchaeology》1993,8(1):75-77

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Do cytotoxic chemotherapy drugs discharged into rivers pose a risk to the environment and human health? An overview and UK case study     

Andrew C. Johnson  Monika D. Jürgens  Richard J. Williams  Klaus Kümmerer  Andreas Kortenkamp  John P. Sumpter 《Journal of Hydrology》2008,348(1-2):167-175

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Climate states and variability of Arctic ice and water dynamics during 1946–1997     

Andrey Y. Proshutinsky  Igor V. Polyakov  Mark A. Johnson 《Polar research》1999,18(2):135-142

Recently observed changes in the Arctic have highlighted the need for a better understanding of Arctic dynamics. This research addresses that need and is also motivated by the recent finding of two regimes of Arctic ice - ocean wind-driven circulation. In this paper, we demonstrate that during 1946-1997 the Arctic environmental parameters have oscillated with a period of 10-15 years. Our results reveal significant differences among atmosphere, ice, and ocean processes during the anticyclonic and cyclonic regimes in the Arctic Ocean and its marginal seas. The oscillating behaviour of the Arctic Ocean we call the Arctic Ocean Oscillation (AOO). Based on existing data and results of numerical experiments, we conclude that during the anticyclonic circulation regime the prevailing processes lead to increases in atmospheric pressure, in ice concentration and ice thickness, river runoff, and surface water salinity - as well as to decreases in air temperature, wind speed, number of storms, precipitation, permafrost temperatures, coastal sea level, and surface water temperature. During the cyclonic circulation regime the prevailing processes lead to increased air and water temperatures, wind speed, number of storms,open water periods, and to decreases in ice thickness and ice concentration, river runoff, atmospheric pressure, and water salinity. The two-climate regime theory may help answer questions related to observed decadal variability of the Arctic Ocean and to reconcile the different conclusions among scientists who have analysed Arctic data obtained during different climate states.  相似文献   

Steady, balanced rates of uplift and erosion of the Santa Monica Mountains, California     

Meigs  Brozovic  & Johnson 《Basin Research》1999,11(1):59-73

Topographic change in regions of active deformation is a function of rates of uplift and denudation. The rate of topographic development and change of an actively uplifting mountain range, the Santa Monica Mountains, southern California, was assessed using landscape attributes of the present topography, uplift rates and denudation rates. Landscape features were characterized through analysis of a digital elevation model (DEM). Uplift rates at time scales ranging from 10⁴ to 10⁶ years were constrained with geological cross-sections and published estimates. Denudation rate was determined from sediment yield data from debris basins in southern California and from the relief of rivers set into geomorphic surfaces of known age. First-order morphology of the Santa Monica Mountains is set by large-scale along-strike variations in structural geometry. Drainage spacing, drainage geometry and to a lesser extent relief are controlled by bedrock strength. Dissection of the range flanks and position of the principal drainage divide are modulated by structural asymmetry and differences in structural relief across the range. Topographic and catchment-scale relief are ≈300–900 m. Mean denudation rate derived from the sediment yield data and river incision is 0.5±0.3 mm yr^?1. Uplift rate across the south flank of the range is ≈0.5±0.4 mm yr^?1 and across the north flank is 0.24±0.12 mm yr^?1. At least 1.6–2.7 Myr is required to create either the present topographic or the catchment-scale relief based on either the mean rates of denudation or uplift. Although the landscape has had sufficient time to achieve a steady-state form, comparison of the time-scale of uplift and denudation rate variation with probable landscape response times implies the present topography does not represent the steady-state form.  相似文献   

A risk assessment approach to contaminants in Port Curtis, Queensland, Australia     

Jones MA  Stauber J  Apte S  Simpson S  Vicente-Beckett V  Johnson R  Duivenvoorden L 《Marine pollution bulletin》2005,51(1-4):448-458

Port Curtis is one of Australia's leading ports for which substantial industrial expansion is proposed over the next decade. However, there has been little attempt to date to assess the extent of contamination in waters, sediments and biota or to characterize the potential impacts of contaminants on aquatic biota. Contaminants of potential concern to biota and human health were investigated in the Port Curtis estuary using a screening-level risk assessment approach. Dissolved metal concentrations in waters were below [ANZECC/ARMCANZ, 2000. Australian and New Zealand Guidelines for Fresh and Marine Water Quality, Vol. 1. The Guidelines, Australian and New Zealand Environment and Conservation Council and Agriculture and Resource Management Council of Australia and New Zealand] trigger values, suggesting low risk of these contaminants. In sediments, arsenic, nickel and chromium concentrations exceeded interim sediment quality guidelines-low (ISQG-low), but were also high in the reference zone suggesting a natural origin. Historical data on naphthalene in Harbour sediments showed that it was also of potential concern. Bioaccumulation of contaminants in a range of biota was also used as an indicator of contaminant exposure. Biota were generally enriched in metals and tributyltin, which was also elevated in water and sediments. Although not unique to Port Curtis, mercury in barramundi was identified as a potential risk to human health.  相似文献   

密歇根地区Ishpeming太古宙绿岩带中矿脉的铅同位素研究     

T.J.Bornhorst  R.I.Thorpe  R.C.Johnson 《地质与资源》1999,8(3):188-192

１　引　言含有石英＋硫化物±碳酸盐矿物组合的矿脉在Ｉｓｈｐｅｍｉｎｇ太古宙绿岩带中普遍存在（见图１）．Ｒｏｐｅｓ金矿的这种矿脉是美国已知的产于太古宙岩石中的唯一有意义的脉状金矿床．Ｒｏｐｅｓ金矿矿脉及伴生的蚀变岩含金约１０５００ｋｇ,其Ａｕ／Ａｇ比值约为０．７．有两度开采史,分别为１８８２～１８９７年和１９８６～１９８９年．产于Ｉｓｈｐｅｍｉｎｇ绿岩带的Ｒｏｐｅｓ矿床和其他小型矿床、矿点中的矿脉为后生成因并受断裂和剪切带控制．矿脉的年龄界定程度很差．本研究的目的,就是以矿脉的方铅矿铅同位素组成为…  相似文献